In a sensational turnaround, the unseeded Frenchman toppled the world number six from Croatia 1-6 6-3 6-2 in a gripping semifinal encounter.
The former world top-10 player, now placed a humble 89th, shrugged off the opening set debacle to stun Cilic, the 2017 Wimbledon finalist, in one hour and 51 minutes.
He will now gun for his 13th ATP title, having played his last final in September 2015 when he lost to compatriot Jo- Wilfred Tsonga at home in Metz.
His consistent returns, even on the powerful serves of Cilic, unsettled the Croat, who started to make unforced errors and could never recover.
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Simon came to the match with a superior 4-1 head-to-head record against Cilic and maintained it. They last played in 2016 at the Miami Masters, where Simon won in a three-setter.
Simon endured a tough 2017 season during which he struggled to reach even quarterfinals on Tour and his ranking plummeted to 89 from 24 at the beginning of the year.
Cilic, as usual, began in an explosive manner firing aces and service winners while Simon struggled to win points on his own serve.
The Frenchman dropped serve at love in the second game and won only two points in the first three games of the match, underlining the dominant start the Croat made.
After being broken at love, Simon saved two break chances in the fourth game but netted a backhand on the third while Cilic held his own to race to a commanding 5-0 lead.
Cilic though remained ruthless and began serving for the set with an 226km ace and closed it with another on his fourth set point.
In a complete turn around, Simon started to return better in the second set.
The Frenchman began to hold his serve and broke Cilic in the second game to take a 3-0 lead in the second set. The break happened in a bizarre manner. At 30-30, Simon found a surprise service return winner as Cilic left the ball, thinking it's going out.
With momentum on his side, Simon began to dominate rallies as he stunned Cilic with some amazing winners and served out the set in the ninth game.
With balls becoming heavier after some hits, Simon mixed it up nicely, sending some short and some deep returns to engage Cilic in long rallies, enticing errors from his opponent.
From 40-15, Cilic lost his serve in the opening game of the decider.
The Croat dropped serve at love in the seventh game and Simon served out the match when Cilic committed a backhand error on second match point.
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